1. Field of Invention
The present invention relates to a method of fabricating a polishing pad. More particularly, the present invention relates to a method of fabricating a polishing pad having a detection window thereon.
2. Description of Related Art
For the semiconductor manufacture of integrated circuits, it is necessary to increase the aspect ratios and the levels of conductive lines in order to increase density of electric components and reduce the production cost. For the wafer with multilevel metalization and/or high aspect ratios, it is complex and difficult to achieve good planarization. Chemical mechanical polishing (CMP) is so far one of the best technologies to achieve global planarization for very large semiconductor integration (VLSI) or even ultra large semiconductor integration (ULSI). This CMP technology was first proposed by the corporation IBM in the USA, and applied in the damascene fabrication of embedded conductive lines.
In general, chemical mechanical polishing relies on the relative movement between the resilient and hard polishing pad and the slurry (small particle suspension), acting on the surface of the wafer for planarization. While the polishing pad moves and presses on the surface of the wafer, the abrasive particles in the slurry are pressed to the surface of the wafer. The friction between the slurry particles and the wafer surface leads to abrasive action (loss) of the wafer, so that the uneven surface of the wafer is planarized and topology of the wafer is refined.
A transparent window is usually designed to be disposed within a specific area of the polishing pad for monitoring the polishing situations. The user can monitor the abrasive situation and control the polishing conditions through the transparent window and the platform setup. Therefore, the transparent window is regarded as the end-point detection of the polishing for the polishing pad.
The prior art method of fabricating the detection window is described in U.S. Pat. No. 5,605,760. In one embodiment, the polishing pad is a transparent sheet made of solid uniform polymer. Alternatively, the polishing pad is opaque and has a transparent window. The transparent window is fabricated by casting a rod or plug of the transparent polymer. This transparent casting is either inserted into the opaque pad before molding or after molding. If the transparent casting is inserted into the polishing pad in mold, because the materials of the polishing pad and the transparent window are different, the contraction difference between the polishing pad and the transparent window may cause the transparent window to be distorted or deformed. Even if the transparent window is inserted into the cut of the polishing pad after molding, the incomplete contact between the polishing pad and the transparent window may cause slurry leakage, thus deteriorating the polishing quality of the wafer.
U.S. Pat. No. 5,893,796 discloses a polishing pad having a transparent window. The transparent window is fabricated by placing a solid transparent insert into the bulk molten polishing pad material, so that the window is integrally molded into the polishing pad. However, the transparent window is made of high polishing resistant materials, while the polishing pad is made of low polishing resistant materials. Therefore, the polishing pad has a higher attrition rate, while the transparent window wears down (polished) at a lower rate, which creates height difference between the polishing pad and the detection window and causes cracking of the polishing pad around the transparent window. This height difference of the polishing pad further leads to non-uniform polishing on the wafer and defects on the surface of the wafer.